An electric optical distance wavelength meter of a phase difference system disclosed in Japanese Published Examined Patent Application No. 3236941 has been often used. FIG. 5 is a block diagram of the electric optical distance wavelength meter.
In the electric optical distance wavelength meter, measurement light L emitted from a light source 3 such as a laser diode is emitted toward a target (prism or the like) 6 placed on a measurement point through a light transmission optical system such as prisms 10, 12, a mirror 4 and an object lens 5. The light source 3 is connected to a modulator 2, and the modulator 2 is connected to a reference signal oscillator 1. The measurement light L is modulated by a reference signal K generated by the reference signal oscillator 1.
The measurement light L reflected on the target 6 is made incident to a detector (light receiving element) 7 such as a photo diode through a light reception optical system composed by the object lens 5 and the mirror 4. Then, the measurement light L is converted into an electric signal as a light receiving signal M by the detector 7. The mutual phase difference of the light receiving signal M and a reference signal K sent from the modulator 2 is measured by a phase meter 9, and a distance to the target 6 can be calculated by the phase difference.
When the above reference signal K has only one frequency, only the half length of the wavelength of the reference signal K can be measured. Consequently, the electric optical distance wavelength meter switches the frequency of the reference signal K sequentially to measure distance at a plurality of frequencies, and integrates the results of the measurements to measure the distance.
On the other hand, the measurement light L emitted from the light source 3 is immediately made incident to the detector 7 as reference light R through prisms 10, 11, and 12 by switching a light path switching device 8. When the distance is measured by using the reference light R as in the measurement light L, an error inherent to the electric optical distance wavelength meter can be known. Thus, the error inherent to the electric optical distance wavelength meter is corrected from the distance measured by using the measurement light L by measuring using the measurement light L and measuring using the reference light R alternately, and thereby the precise distance to the target 6 can be calculated.
In addition, the electric optical distance wavelength meter of a pulse traveling time system disclosed in Japanese Published Unexamined Patent Application No. H01-213592 to be described below has also been known. In the pulse traveling time system, pulse light is emitted toward the target, and the pulse light reflected from the target is received. The distance to the target is measured from the time for reciprocating to the target of the pulse light.